School of Molecular Biosciences, Washington State University, Pullman, Washington, USA.
Environ Mol Mutagen. 2024 Apr;65 Suppl 1(Suppl 1):14-24. doi: 10.1002/em.22569. Epub 2023 Aug 25.
Exposure to ultraviolet (UV) light is the primary etiological agent for skin cancers because UV damages cellular DNA. The most frequent form of UV damage is the cyclobutane pyrimidine dimer (CPD), which consists of covalent linkages between neighboring pyrimidine bases in DNA. In human cells, the 5' position of cytosine bases in CG dinucleotides is frequently methylated, and methylated cytosines in the TP53 tumor suppressor are often sites of mutation hotspots in skin cancers. It has been argued that this is because cytosine methylation promotes UV-induced CPD formation; however, the effects of cytosine methylation on CPD formation are controversial, with conflicting results from previous studies. Here, we use a genome-wide method known as CPD-seq to map UVB- and UVC-induced CPDs across the yeast genome in the presence or absence in vitro methylation by the CpG methyltransferase M.SssI. Our data indicate that cytosine methylation increases UVB-induced CPD formation nearly 2-fold relative to unmethylated DNA, but the magnitude of induction depends on the flanking sequence context. Sequence contexts with a 5' guanine base (e.g., GCCG and GTCG) show the strongest induction due to cytosine methylation, potentially because these sequence contexts are less efficient at forming CPD lesions in the absence of methylation. We show that cytosine methylation also modulates UVC-induced CPD formation, albeit to a lesser extent than UVB. These findings can potentially reconcile previous studies, and define the impact of cytosine methylation on UV damage across a eukaryotic genome.
紫外线（UV）暴露是皮肤癌的主要病因，因为 UV 会破坏细胞 DNA。最常见的 UV 损伤形式是环丁烷嘧啶二聚体（CPD），它由 DNA 中相邻嘧啶碱基之间的共价键组成。在人类细胞中，CG 二核苷酸中胞嘧啶碱基的 5'位置经常被甲基化，TP53 肿瘤抑制基因中的甲基化胞嘧啶经常是皮肤癌中突变热点的位点。有人认为，这是因为胞嘧啶甲基化促进了 UV 诱导的 CPD 形成；然而，胞嘧啶甲基化对 CPD 形成的影响存在争议，之前的研究结果相互矛盾。在这里，我们使用一种称为 CPD-seq 的全基因组方法，在体外由 CpG 甲基转移酶 M.SssI 甲基化的存在或不存在的情况下，在酵母基因组中绘制 UVB 和 UVC 诱导的 CPD。我们的数据表明，与未甲基化的 DNA 相比，胞嘧啶甲基化使 UVB 诱导的 CPD 形成增加了近 2 倍，但诱导的幅度取决于侧翼序列的上下文。具有 5'鸟嘌呤碱基的序列上下文（例如 GCCG 和 GTCG）由于胞嘧啶甲基化而显示出最强的诱导作用，这可能是因为在没有甲基化的情况下，这些序列上下文形成 CPD 损伤的效率较低。我们表明，胞嘧啶甲基化也调节 UVC 诱导的 CPD 形成，尽管不如 UVB 强烈。这些发现有可能调和以前的研究，并定义胞嘧啶甲基化对真核基因组中 UV 损伤的影响。
